1. Field of the Invention
The present invention relates to a cylinder head structure for an engine.
2. Description of the Related Art
Referring to FIG. 8 of the accompanying drawings, illustrated is a plan view of a cylinder head a of a four-valve center-nozzle type diesel engine. This engine has four cylinders in series (not shown). As illustrated, the cylinder head a includes a plurality of block portions b for a plurality of cylinders of the engine respectively. Each block portion b defines a plurality of valve tappet guide holes d to receive valve tappets (not shown) respectively. The cylinder head a also includes seat portions c on which heads of cylinder head bolts seat when the cylinder head bolts are tightened to join the cylinder head with a cylinder block (not shown).
Each block portion b has four valve tappet guide holes d formed in a top surface e (indicated by the oblique lines) of the cylinder head a for each cylinder, and a single fuel injection nozzle hole f at the center of the four tappet guide holes d for each cylinder. Cylindrical valve tappets are received in the guide holes d such that they can move in axial directions of respective valves (intake or exhaust valves: not shown). Each of the valve tappets has a top surface which contacts an associated cam of a cam shaft (not shown) and has a bottom surface which abuts an upper end of a valve shaft (stem) of the associated valve, so that it forces the valve shaft downward and upward to open and close the associated valve upon rotations of the cam shaft.
Each of the seat portions c has a bolt hole g into which an associated cylinder head bolt inserted. The cylinder head bolts are inserted into the bolt holes g from the top surface e of the cylinder head a, and lower end stem portions are threaded into female threads formed in the cylinder block until bolt heads seat on seating surfaces h of the seat portions c. These head bolts firmly unite the cylinder head a with the cylinder block.
The seating surfaces h for the cylinder head bolts are coplanar to the top surface e of the cylinder head a so that they are substantially continuous to the top surface i of the block portions b. Further, six seating surfaces h surround one block portion b for each cylinder at predetermined intervals such that they are arranged around an associated cylinder bore in the plan view.
When the head blots are inserted in the bolt holes g and tightened to join the cylinder head a to the cylinder block, axial tensions of the head bolts press down the seating surfaces h by the bolt heads and therefore radiant tensile forces F are generated in the top surfaces i of the block portions b as indicated by the arrows in FIG. 8 since each of the block portions b is surrounded by the six seating surfaces h. In addition, the cylinder head a is subjected to a pressure as combustion takes place in the cylinders, but it is restricted by the cylinder head bolts (specifically at the bolt heads and seating surfaces h), so that similar tensile forces F are also generated in the top surfaces i of the block portions b by the combustion pressure.
These tensions F are concentrated to the top surfaces i including a shallow area from the top surfaces i. As the tensions F are produced in the top surfaces i of the block portions b for the respective cylinders, they are concentrated to the thinnest or narrowest interportions j between the tappet guide holes d and nozzle holes f. This would cause cracking k in the thin interportions j. The cracking k likely occurs if the cylinder head a is made of an aluminum or its alloy. The engines are often made of the aluminum to reduce the engine weight.